Multi-hop time reservation using adaptive control for energy efficiency

ABSTRACT

The present invention extends the TRACE (Time Reservation Using Adaptive Control for Energy Efficiency) protocol to allow multi-hop communication in dispersed radio networks. The nodes in the network are dynamically clustered based on proximity and interference level. Time is divided into superframes, each cluster choosing a frame within the superframe for data transmission to reduce the likelihood of interference.

SUMMARY OF THE INVENTION

The present invention is directed to technique for cluster formation andtime reservation in networks such as radio networks and morespecifically to a technique for use in multi-hop communication.

DESCRIPTION OF RELATED ART

In a wireless radio network, the transmission medium is common to allradios, and the MAC protocol controls access to this shared medium. Theobjective of controlled access is to avoid simultaneous transmissionattempts (that will result in collisions) while maintaining the maximalthroughput, minimal energy, and bounded packet delay of the wholenetwork.

MAC protocols can be classified into two main categories: centralizedand distributed. In a distributed MAC protocol, radios communicatewithout a central controller or base station. In other words, everyradio should create its own access to the medium through a predeterminedset of rules (e.g., IEEE 802.11). A centralized MAC protocol, on theother hand, has a controller node or a base station that is the maestroof the network (e.g., Bluetooth). All the nodes in the network accessthe medium through some kind of schedule determined by the controller.

Although distributed MAC protocols are deemed as the best existingsolutions for multi-hop networks, they are not favorable in single-hopnetworks because they do not perform as well as centralized MACprotocols in single hop networks. In addition, centralized MAC protocolsare generally more deterministic than distributed MAC protocols, whichis a desirable feature for real-time traffic with delay constraints. Asa result, it is advantageous to use a centralized MAC protocol in asingle hop network that supports real-time traffic delivery. Forexample, in an IEEE 802.11 network, all nodes should be active all thetime, because they do not know when the next transmission is going totake place. However, in a Bluetooth network, nodes can enter sleep modefrequently due to the explicit polling of the slave nodes by the masternode, which is an effective method to save power. Moreover, the IEEE802.11 protocol cannot guarantee bandwidth or delay constraints or fairmedium access. In fact, all of these parameters are functions of thedata traffic, and they become unpredictable and often unacceptable athigh data rates. However, some centralized algorithms can guarantee someof the above requirements within certain ranges by making use ofcoordination via scheduling.

A centralized MAC protocol can work with TDMA, FDMA, CDMA or hybridphysical layers. Among them, TDMA is the most appropriate choice forpower savings because radios can go to sleep mode whenever they are notinvolved with data transmission/reception. In a centralized TDMA typeMAC protocol, the two most important issues are the controllerassignment and the data transmission schedule, which correspond to thecoordinator and the coordination, respectively. The coordinator could bea fixed predetermined radio, which is the sole controller for the entirenetwork lifetime. The main drawback of this approach is that wheneverthe controller dies, the whole network also dies. The controllerdissipates more energy than other nodes because of its additionalprocesses and transmissions/receptions. Because of this higher energydissipation, most possibly the controller will run out of energy beforeall the other nodes, leaving the entire network inoperable for the restof the network lifetime, even though many other remaining nodes haveenough energy to carry on transmissions/receptions. The datatransmission schedule could also be fixed, but this does not allow thesystem to adapt to dynamic environments such as nodes entering thenetwork. The alternative approach to a fixed controller and schedule isdynamic coordinator switching and schedule updating, which is a remedyfor the problems described above. However, this approach comes with itsown problems: overhead in controller handover and increased overhead inthe schedule updates.

The information content in a broadcast medium may be higher than theusable range of a single node, in which case the nodes should select toreceive only certain data packets. The straightforward approach, whichis listening to all data transmissions, keeping the ones desired, anddiscarding the others, is a highly inefficient way of discriminatingdata. An energy efficient method is information summarization beforedata transmission.

The inventors have previously developed a MAC protocol called TRACE(Time Reservation using Adaptive Control for Energy Efficiency). TRACEis described in B. Tavli and W. B. Heinzelman, “TRACE: Time ReservationUsing Adaptive Control for Energy Efficiency,” to appear in IEEE J.Select. Areas Comm., 2002, whose disclosure is hereby incorporated byreference in its entirety into the present disclosure. TRACE usesdynamic coordinator switching and schedule updating to adapt to achanging environment and reduce energy dissipation in the nodes. Otherfeatures of TRACE, such as information summarization, data streamcontinuation monitoring, multi-level coordinator backup, priority basedchannel access, and contention for channel access reinforce theenergy-efficiency, reliability, bounded delay, and maximized throughputof the network.

TRACE is an energy-efficient dynamic TDMA protocol designed forreal-time data broadcasting. In TRACE, data transmission takes placeaccording to a dynamically updated transmission schedule. Initial accessto data slots is through contention, but once a node reserves a dataslot, its reservation for a data slot in the subsequent frames continuesautomatically as long as the node continues to broadcast a packet ineach frame. Thus, nodes only need to contend for data slots at thebeginning of data bursts.

A controller in the network is responsible for creating the TDMAschedule based on which nodes have continued reservations from previousframes and which have successfully contended for data slots in thecurrent frame. The controller transmits this schedule to the rest of thenodes in the network at the beginning of the data sub-frame. Wheneverthe energy of the controller drops below the energy level of the othernodes in the network by more than a set amount, it assigns another radiowith higher energy than itself as the next controller. Controllerhandover takes place during the TDMA schedule transmission by specifyingthe ID of the new controller. Finally, if the number of transmissions ina frame exceeds a predetermined threshold, each node listens only todata from certain other nodes. Each node determines which transmittersto listen to based on information obtained from all the nodes during theinformation summarization (IS) slot.

TRACE is therefore well suited to fulfill the tactical communicationrequirements of a squadron of soldiers that are all in communicationrange of each other. However, often soldiers will be scattered over alarge area, requiring multi-hop communication rather than point-to-pointcommunication. As yet, no solution exists in the art to extend TRACE tosuch a situation.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to extend TRACE to situationsin which multi-hop communication is required.

To achieve the above and other objects, the present invention isdirected to an extension of TRACE for such multi-hop networks, known asMulti-Hop TRACE, or MH-TRACE. In MH-TRACE, the network is dynamicallypartitioned into clusters, which are maintained by clusterheads. Time isdivided into super frames consisting of several frames, and each clusterchooses a frame during which nodes in the cluster can transmit data.Each frame includes a contention sub-frame, a summarization sub-frame,and a data sub-frame. Nodes transmit their initial channel accessrequests to their clusterhead in the contention sub-frame. As long as anode uses its reserved data slot, its channel access is renewed insubsequent frames. Nodes that are granted channel access through thetransmission schedule transmitted by the clusterhead transmit asummarization packet prior to actual data transmission in thesummarization sub-frame. Thus, each node knows the future datatransmissions in its receive range by listening to the summarizationpackets. Nodes save energy by entering sleep mode whenever they do notwant to be involved with a packet transmission or reception.

New cluster creation and existing cluster maintenance are designed tominimize interference between clusters that choose the same frame fortransmission. Unlike existing clustering approaches, the clusteringscheme according to the present invention is not based on connectivityinformation. In addition, we do not use frequency division or codedivision to avoid inter-cluster interference in order to enable eachnode in the network to receive all the desired data packets in itsreceive range, not just those from nodes within the same cluster. Thus,our clustering approach does not create hard clusters—the clustersthemselves are only used for assigning time slots for nodes to transmittheir data. Instead, each node creates its own soft listening cluster bychoosing to listen to certain nodes in its receive range. Our clustercreation and maintenance algorithms do not bring high overhead to thenetwork, and cluster stability is preserved, even in the presence ofnode mobility.

The invention provides a distributed MAC protocol for energy efficientvoice packet broadcasting in a multi-hop radio network. In MH-TRACE, thenetwork is dynamically partitioned into clusters by a distributedalgorithm run by each radio, which does not use any global informationexcept global clock synchronization. The clustering algorithm is simpleand robust enough to ensure that the gain from clustering is much higherthan the overhead required for cluster creation and maintenance, even inthe presence of the node mobility.

In MH-TRACE, time is organized into superframes, which consist ofseveral time frames. Each cluster chooses a frame for transmittingcontrol packets and for the transmission of data from nodes in thecluster. The chosen frame is the frame with least interference, asmeasured by the node forming the cluster. However, each node in thenetwork can receive all the desired packets in its receive range withoutany restriction based on the formed clusters. Each node learns aboutfuture data transmissions in its receive range from informationsummarization (IS) packets sent prior to data transmission in adesignated time period by each transmitting node. Therefore, each nodecreates its own listening cluster and receives the packets it wants. Byavoiding energy dissipation for receiving unwanted data packets or forwaiting in idle mode, MH-TRACE guarantees the network to be highlyenergy efficient, approaching the theoretical minimum energy.Furthermore, since data transmission is contention free, the throughputof MH-TRACE is better than the throughput of CSMA type protocols underhigh traffic loads.

When compared to CSMA-type broadcast protocols like IEEE 802.11,MH-TRACE has three advantages: (i) energy efficiency due to the use ofTDMA and data summarization slots, which allow nodes to enter sleep modeoften, (ii) higher throughput due to the coordinated channel access, and(iii) support for real-time operation due to its time-frame based cyclicoperation. We ran ns-2 simulations to investigate the performance ofMH-TRACE and to compare it with IEEE 802.11. Initial results show thatour dynamic clustering approach based on interference level createsrobust clusters, and MH-TRACE performs better than IEEE 802.11 in termsof energy efficiency and throughput.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will be set forth indetail with reference to the drawings, in which:

FIG. 1 shows a format of a super frame used in the preferred embodiment;

FIG. 2 shows a format of a frame contained in the super frame of FIG. 1;

FIG. 3 shows a flow chart of a cluster creation algorithm according tothe preferred embodiment;

FIG. 4 shows a flow chart of a cluster maintenance algorithm accordingto the preferred embodiment;

FIG. 5 shows a distribution of nodes at 0.050 sec;

FIG. 6 shows a distribution of the same nodes at 100 sec; and

FIG. 7 shows a schematic diagram of a device usable as a node in thepreferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be set forth indetail with reference to the drawings.

MH-TRACE is organized around super frames with duration T_(SF) matchedto the periodic rate of voice packets. As shown in FIG. 1, each superframe 102 consists of N_(F) frames 104, each having a duration T_(F).

The frame format is presented in FIG. 2. Each frame 104 consists of twosub-frames: a control sub-frame 202 and a data sub-frame 204. Thecontrol sub-frame 202 consists of a beacon slot 206, a clusterheadannouncement (CA) slot 208, a contention slot 210, a header slot 212 andan information summarization (IS) slot 214.

At the beginning of each occupied frame, the clusterhead transmits abeacon message. This is used to announce the existence and continuationof the cluster to the cluster members and the other nodes in thetransmit range of the clusterhead. By listening to the beacon and CApackets, all the nodes in the carrier sense range of this clusterheadupdate their interference level table. The contention slot, whichimmediately follows the CA slot, consists of N_(c) sub-slots. Uponhearing the beacon, each node that has data to send but did not reservea data slot in the previous cyclic frame, randomly chooses a sub-slot totransmit its request. If the contention is successful (i.e., nocollisions), the clusterhead grants a data slot to the contending node.Following the contention subslot, the clusterhead sends the header,which includes the data transmission schedule of the current frame. Thetransmission schedule is a list of nodes that have been granted dataslots in the current frame, along with their data slot numbers. Acontending node that does not hear its ID in the schedule understandsthat its contention was unsuccessful (i.e., a collision occurred or allthe data slots are already in use) and contends again in the followingframe. If the waiting time for a voice packet during contention forchannel access exceeds the threshold, T_(drop), it is dropped.

The IS slot begins just after the header slot and consists of N_(D)sub-slots. Nodes that are scheduled to transmit in the data sub-frametransmit a short IS message exactly in the same order as specified bythe data transmission schedule. An IS message has an end-of-stream bit,which is set to one if the node has no data to send. Each receiving noderecords the received power level of the transmitting node and insertsthis information into its IS table. The IS info table is used as aproximity metric for the nodes (i.e., the higher the received power, theshorter the distance between transmitter and receiver nodes). Nodes thatare not members of this cluster also listen to the IS slot and recordthe received power level. Each node creates its own listening cluster byselecting the top N_(max) transmissions that are the closesttransmitters to the node. Note that other methods of deciding whichnodes to listen to can be used within the TRACE framework by changingwhat data nodes send in the IS slot. Each node does not necessarily needto listen to a predetermined number of closest nodes—this can be chosendynamically based on what data is being transmitted. Furthermore, thereceive clusters can be based on information besides proximity. Forexample, if each node sends information about the data packet in the ISslot, nodes can choose their listening clusters based on whether or notthey want to receive the packet described in the IS slot. Priorityinformation can be transmitted in the IS slot as well, so nodes know toalways listen to high priority data. Hence the network is softlypartitioned into many virtual clusters based on the receivers; this isfundamentally different from transmitter based network partitioning.

The data sub-frame is broken into constant length data slots. Nodeslisted in the schedule in the header transmit their data packets attheir reserved data slots. Each node listens to at most N_(max) datatransmissions in a single super frame; therefore, each node is on for atmost N_(max) data slots.

A node keeps a data slot once it is scheduled for transmission as longas it has data to send. A node that sets its end-of-stream bit (in theIS packet) to one because it has no more data to send will not begranted channel access in the next frame (i.e., it should contend to geta data slot once it has new data to send). Automatic renewal of dataslot reservation enables real-time data streams to be uninterrupted.

FIG. 3 shows a flow chart of a cluster creation algorithm. At theinitial startup stage of step 302, a node listens to the medium in step304 to detect any ongoing transmissions for the duration of one superframe time, T_(SF), to create its interference table in step 306 foreach frame within the super frame. If there is already a clusterhead inits receive range, as determined in step 308, the node starts its normaloperation. If it is determined in step 310 that more than one beacon isheard, the node that sent the beacon with higher received power ischosen as the clusterhead in step 312 (i.e., the closest clusterhead ischosen). Either way, the node joins a cluster in step 314.

If no beacon is detected in step 308, then the node checks theinterference level of the least noisy frame in step 316 to see if theinterference level is low enough to create a new cluster. If theinterference level is low enough to create a new cluster, then the nodepicks a random time or the least noisy frame to transmit its own beaconsignal in step 318, starts its contention timer in step 320, and beginsto listen to the channel until its contention timer expires. If it isdetermined in step 322 that a beacon is heard in this period, then thenode just stops its timer and starts normal operation; that is, it waitsfor a header in step 324 and joins the cluster in step 326. Otherwise,when the timer expires, the node sends the beacon in step 328 andassumes the clusterhead position. It is determined in step 330 whether abeacon or a header is heard. If a header is received, the node joins acluster in step 336. If a beacon is heard, the node goes to step 324. Ifneither a beacon nor a header is heard, the node sends a header in step332 and creates a cluster in step 334.

In case there is a beacon collision, none of the colliding nodes willknow it, but the other nodes hear the collision, so the initial setupcontinues. All the previously collided nodes, and the nodes that couldnot detect the collision(s) because of capture, will learn of thecollisions with the first successful header transmission.

Finally, if it is determined in step 316 that the node does not hear anybeacons but the interference level is higher than the maximum level tostart a new cluster, then the node is blocked from any transmissions.However, it can still receive all the packets in its receive range. Thenode listens for a time, e.g., equal to twice the superframe time instep 336 and then goes back to step 308. The reason for node blocking isthat if a new cluster centered at the high interference region iscreated, then packet transmissions from the multiple clusterstransmitting at the same time frame will collide at some locations withhigh probability. Thus, blocking new cluster creation is preferable overletting new clusters be formed with very high interference. Furthermore,if nodes are mobile, a blocking situation will only be temporary, andthe node will be able to transmit data as soon as either it moves intorange of an existing clusterhead or it moves far enough away fromexisting clusterheads that the node can start a new cluster withoutinterfering with existing clusters.

Each clusterhead continuously records the interference level of eachframe by listening to the beacon transmission and CA transmission slots,which are at the beginning of each frame. Since only the clusterheadsare allowed to transmit in these slots, it is possible for eachclusterhead to measure the received power level from other clusterheadsand know the approximate distances to other clusterheads in the carriersense range. A clusterhead can record the interference level of eachframe by listening to the beacon slot, but the beacon slot becomesuseless for a clusterhead's own frame, because it is transmitting itsown beacon. CA transmission is used to determine the interference levelof the co-frame clusters (i.e., clusters that have chosen to broadcastover the same frame). This is done by having each node transmit the CApacket with a probability p_(CA). If this probability is set to 0.5,then each clusterhead records the interference level in its frame, onthe average, at 4T_(SF) time. It is possible to use two consecutive CAslots, which has an average settling time of 2T_(SF).

A cluster maintenance protocol will be explained with reference to theflow chart of FIG. 4. A clusterhead keeps its frame (the steady stateoperation of step 402) unless the interference threshold becomes toohigh (step 404 or 406) or any other clusterhead enters in its receiverange (step 408). A cluster leaves a frame with high interference instep 410 and moves to a low interference frame with probability p_(CF).The reason for adding such randomness is to avoid the simultaneous andunstable frame switching of co-frame clusters, which are theinterference source for each other. In case of high interference in allframes, the clusterhead resigns in step 412 with a probability p_(HI).If this probability is set to 0.5, then the probability that only one ofthe two clusterheads resigns becomes 0.67. When two clusterheads enterin each other's receive range, the one who receives the other's beaconfirst resigns directly in step 414.

If a node does not receive a beacon packet from its clusterhead forT_(NB) time, either because of mobility of the node or the clusterheador the failure of the clusterhead, then it enters the initial startupprocedure.

It is possible that network node distribution is not perfectly uniformor traffic at a specific portion of the network is higher than the otherregions, especially at the regions close to the network center wherenode density is higher. In addition, node distribution changescontinuously in time. This creates clusters with few nodes andunderutilization of the channel. Some nodes are in the transmit range ofmore than one clusterhead, and they choose to be a member of the clusterwith the closer clusterhead. For these nodes, if all the data slots inthe cluster that they belong to are in use and another cluster in rangehas available data slots, they can contend for channel access from thefurther clusterhead rather than the one that is closer. Another optionis to select the closest clusterhead that has available data slots. Byincorporating this dynamic channel allocation scheme into MH-TRACE, onemore degree of freedom is added to the network dynamics, which reducesthe adverse affects of clustering.

Network-wide multi-hop broadcasting is an operation that drains networkresources and leaves the network inoperable very quickly under evenmoderate traffic. Moreover, voice packets with delay limits cannottraverse more than several hops before being dropped. Thus, only aselected subset of the data packets is broadcasted to multiple hops inMH-TRACE.

MH-TRACE supports an optional prioritized operation mode. In this mode,nodes have three pre-assigned priority levels, of which Priority Level-1(PL1) is the highest priority and PL3 is the lowest priority. Thehighest level has the highest QoS (quality of service), and the lowestlevel has the lowest QoS. PL1 and PL2 nodes get channel access in allsituations. If all the data slots are already in use, reservation forsome of the PL3 nodes are taken away and higher priority nodes aregranted channel access.

All the nodes should listen to data from PL1 nodes, whether or not theyare close to the nodes. However not all the nodes that belong to thesame cluster can hear each other directly, so the clusterhead which canbe heard by all the nodes in the cluster, rebroadcasts packets from PL1nodes. Nodes that can hear more than one clusterhead also forward thePL1 packets they receive, so that other clusterheads can receive thesepackets. In this way, packets from PL1 nodes will be transmitted to alarge number of nodes throughout the network. As an alternative,information can be transmitted between at least two of the clusterheadsby way of at least two nodes, one of which is communicating with onecluster and the other of which is communicating with the other cluster,if those at least two nodes are in communication with each other.

Each node creates its receiver-based listening cluster, which has amaximum of N_(max) members, by choosing the closest nodes based on theproximity information obtained from the received power from thetransmissions in the IS slots. Priority has precedence over proximity;therefore, transmissions by PL1 nodes are always included in thelistening cluster by removing the furthest node in the cluster. To avoidinstantaneous changes in the listening clusters and to make them morestable, there is also a continuity rule: a member of the listeningcluster cannot be excluded from the listening cluster until it finishesits talk spurt, which is a natural extension in the sense that if aspeech stream is broken in the middle, the whole transmission becomesuseless.

To test the performance of MH-TRACE, we started conducting simulationsusing the ns software package. We simulated conversational voice codedat 32 Kbps. The channel rate is chosen as 2 Mbps. The transport agentused in the simulations is very similar to UDP, which provides besteffort service. Acronyms, descriptions and values of the parameters usedin the simulations are presented in Table I below.

TABLE I Acro- nym Description Value T_(SF) Super Frame duration 24360 μsN_(F) Number of frames within a super frame 7 T_(F) Frame duration 3480μs T_(CSF) Control sub-frame duration 560 μs T_(DSF) Data sub-frameduration 2968 μs T_(B) Beacon slot duration 24 μs T_(CA) ClusterheadAnnouncement slot duration 40 μs T_(CS) Contention slot duration 260 μsT_(C) Contention sub-slot duration 24 μs T_(H) Header slot duration 80μs (max) T_(ISS) Information summarization slot duration 168 μs T_(IS)Information summarization sub-slot duration 24 μs T_(D) Data slotduration 424 μs IFS Inter-frame space 8 μs T_(drop) Packet dropthreshold 50.0 ms N_(D) Number of data slots 7 N_(C) Number ofcontention sub-slots 9 P_(T) Transmit power 0.6 W P_(TE) Transmitelectronics power 0.318 W P_(PA) Power amplifier power 0.282 W P_(R)Receive power 0.3 W P_(I) Idle power 0.1 W P_(S) Sleep power 0.0 W m_(s)Average spurt duration 1.0 s m_(g) Average gap duration 1.35 s

Super frame time is 24360 μs, consisting of seven frames (i.e.,N_(F)=7). Frame time, T_(F), is 3580 μs; of this 2968 μs is for the datasub-frame, DSF, and 560 μs is for the control sub-frame, CSF. There are9 24 μs duration contention sub-slots, 7 24 μs duration IS sub-slots,and 7 424 μs duration data slots. Beacon, CA, contention, and IS packetsare all 4 bytes. The header packet has a variable length of 4-18 bytes,consisting of 4 bytes of packet header and 2 bytes of data for each nodeto be scheduled. Data packets are 104 bytes long, consisting of 4 bytesof packet header and 100 bytes of data. Each slot or sub-slot includes 8μsec of guard band (IFS) to account for switching time and round-triptime.

For voice source modeling, we assume each node has a voice activitydetector, which classifies speech into “spurts” and “gaps” (i.e., gapsare the silent moments during a conversation). During gaps, no datapackets are generated, and during spurts, data packets are generated inthe rate of the speech coder, which is 32 Kbps in our case. Both spurtsand gaps are exponentially distributed statistically independent randomvariables, with means m_(s) and m_(g), respectively. In our simulationsand analysis we used the experimentally verified values of m_(s) andm_(g), which are 1.0 s and 1.35 s, respectively.

We used the energy model, where transmit power consists of a constanttransmit electronics part, P_(T)E, and a variable power amplifier part,P_(PA). Hence the transmit power, P_(T), can be expressed as the sum oftwo termsP _(T) =P _(TE) +P _(PA)   (1)P_(PA) should be adjusted to compensate for the path loss in wavepropagation. The propagation model is a hybrid propagation model, whichassumes d² power loss for short distances and d⁴ power loss for longdistances. Receive power, P_(R), is dissipated entirely on receiverelectronics. Idle power, P_(I), is the power needed to run theelectronic circuitry without any actual packet reception. In sleep mode,the radio is just shut down so sleep mode power, P_(S), is very low.

We used the random way-point mobility model to create mobilityscenarios. FIG. 5 shows a snapshot of the distribution of 100 nodes overa 750 m by 750 m area at 0.050 s. FIG. 6 shows the snapshot at 100.0 s.Node speed is a uniform random variable between 0.0 m/s and 5.0 m/s(average speed of a marathon runner). The circles around the nodes showthe clusterheads. We ran the simulation for 100.0 s. The number ofclusterheads throughout the entire simulation time is 20, and theaverage number of blocked nodes is 0.58 nodes/frame.

In MH-TRACE, each node contains the functionality to perform theoperations described above, either as a clusterhead or as merely a nodeas required. A block diagram of a radio device capable of functioning asan MH-TRACE node is shown in FIG. 7. The radio device 700 includes anantenna 702 and circuitry 704 for transmitting and receiving under thecontrol of a processor 706. A non-volatile memory 708 includes thesoftware for permitting the processor 706 to perform the requiredoperations. Of course, other components can be included, based on thesort of network to be implemented, e.g., equipment for a voice network,a video/voice network, etc.

MH-TRACE is energy-efficient when compared to existing MAC protocols,like IEEE 802.11. Under low to medium traffic load, both MH-TRACE and802.11 have similar throughput characteristics, but under high traffic,MH-TRACE performs better.

The clustering approaches proposed in the literature are mostly linklevel algorithms, which create clusters based on connectivityinformation, which changes quickly, thus forcing the network to createand destroy clusters very fast. This creates high overhead on a mobilenetwork and annuls the gain obtained from clustering. In MH-TRACE, onthe other hand, cluster formation can be completely based on MAC layerinformation; cluster creation, termination, and maintenance do not bringmuch overhead to the network.

The most important advantage of MH-TRACE is that it achieves trafficadaptive energy efficiency in a multi-hop network without using anyglobal information except synchronization. We used the cluster conceptin such a way that each node creates its own listening cluster as if itis operating under a CSMA type protocol. However, collisions of datapackets are also avoided by means of coordination via scheduling. Thus,advantageous features of fully centralized and fully distributednetworks are combined to create a hybrid and better protocol forreal-time energy efficient broadcasting in a multi-hop network withoutmaking any assumptions about global knowledge.

While a preferred embodiment has been set forth above, those skilled inthe art will readily appreciate that other embodiments can be realizedwithin the scope of the invention. For instance, the clusteringtechnique is not limited to voice networks, but can be implemented withany suitable network. As an example, a group of hearing-impaired personscan have communication devices with cameras and low-resolution screenslarge enough to display sign language intelligibly, possibly withseveral panels. Given the ability to transmit image data at a sufficientrate, e.g., with MPEG compression, the devices can be linked throughMH-TRACE to permit visual communication. Furthermore, numerical valuesare illustrative rather than limiting; those skilled in the art who havereviewed the present disclosure will readily appreciate that othernumerical values can be implemented as needed. Therefore, the presentinvention should be construed as limited only by the appended claims.

1. A method for transmitting information in a network having a pluralityof nodes which are grouped into clusters, the method comprising: (a)dividing time into superframes, each superframe comprising apredetermined number of frames, each frame comprising (i) a beacon slotfor sending a beacon, (ii) a plurality of contention slots for sendingrequests for data slots, (iii) a header slot for sending a header whichcomprises a transmission schedule for the frame, (iv) a plurality ofinformation summarization slots for sending summaries of theinformation, and (v) a plurality of data slots, each for informationtransmission by one of the nodes; (b) permitting each of the clusterswithin the network to choose one of the predetermined number of frames;(c) transmitting information within each of the clusters during said oneof the predetermined number of frames in each superframe; and (d)transmitting information between at least two of the clusters by way ofone or more nodes; wherein step (c) comprises: (i) in each of theframes, permitting each of the plurality of nodes to reserve one of theplurality of data slots by sending the requests in the contention slots;and (ii) maintaining a reservation of each of the plurality of dataslots which is reserved by one of the plurality of nodes until said oneof the plurality of nodes indicates that it has stopped transmitting;and wherein: each node sends an information summarization packet duringa corresponding one of the information summarization slots; and eachnode indicates that it has stopped transmitting by sending anend-of-transmission bit in its information summarization packet.
 2. Themethod of claim 1, wherein: each node records a power level of theinformation summarization packet sent by each of a plurality of othernodes to determine a listening cluster of nodes based at least partiallyon proximity; and said each node listens to the nodes in the listeningcluster.
 3. The method of claim 2, wherein: each node receives theinformation summarization packets sent by the nodes in its listeningcluster, determines in accordance with the information summarizationpackets which information it wishes to receive, and listens only to thenodes sending the information that it wishes to receive.
 4. The methodof claim 3, wherein, when a node is not transmitting information orlistening, the node enters sleep mode.
 5. The method of claim 1,wherein, when a node sends the end-of-transmission bit, that node losesits reserved data slot.